Moro's power draining made him able to defeat Super Saiyan Blue Goku and Vegeta. But in the fight with Majin Buu there isnt any sign they are working at all. Why Moro's power draining dont work in Majin Buu?
There is no information as of now regarding this question. However it is safe to assume that there may be some flaws in Moro's draining power. The way or who the power could be absorbed from may play a part in this.